Porous polymeric structures having pores in the range of 0.01 to 10 microns are commonly used for microfiltration. Such membrane structures may be prepared from thermoplastic polymers using precipitation techniques and formed into various shapes including hollow fibres or flat sheets.
The thermal precipitation technique for membrane formation commences with the formation of a solution of a thermoplastic polymer in a solvent at an elevated temperature. The solution is then cooled and, at a specific temperature which depends on the polymer, the solvent, the concentration of the polymer in the-solvent and the rate of cooling, phase separation occurs and the liquid polymer separates from the solvent.
All practical precipitation methods follow the same general process which is reviewed by Smolders et al in Kolloid Z.u.Z. Polymere 43, 14-20 (1971). The article distinguishes between spinodal and binodal decomposition of a polymer solution.
When the solution of a polymer in a solvent is allowed to cool at an infinitely slow rate, a temperature is reached, below which phase separation occurs and the liquid polymer separates from the solvent. This is called binodal decomposition of the polymer solution.
When the rate of cooling is finite, the temperature at which phase separation occurs is generally lower than in the case of binodal decomposition. This is called spinodal decomposition of the polymer solution.
For practical purposes, all precipitation processes must be reasonably fast and so fall into the category of spinodal decomposition.
In most processes, the relative polymer and solvent concentrations are such that phase separation results in fine droplets of solvent forming in a continuous polymer phase. These fine droplets form the pores of the membrane. As cooling continues, the polymer freezes around the solvent droplets. When phase separation occurs there is still some solubility of the polymer in the solvent and solvent in the polymer.
As the temperature is lowered, these solubilities decrease, and more and more solvent droplets appear in the polymer matrix. Crystallization of the droplets within the polymer results in shrinkage and cracking, thus forming interconnections between the pores. Finally, the solvent is removed from the pores.
Known precipitation methods of porous membrane formation depend on the liquid polymer separating from the solvent followed by cooling so that the solidified polymer can then be separated from the solvent. Whether the solvent is liquid or solid when it is removed from the polymer depends on the temperature at which the operation is conducted and the melting point of the solvent.
True solutions require that there be a solvent and a solute. The solvent constitutes a continuous phase, and the solute is distributed randomly in the solvent at a molecular level. Such a situation is almost unknown with polymer solutions. Long polymer chains tend to bend back on themselves and form temporary interactions or bonds with other polymer chains with which they come into contact. These interactions are continually forming and breaking, and new ones are formed. Polymer solutions are thus rarely true solutions but lie somewhere between true solutions and mixtures.
In many cases it is also difficult to state which is the solvent and which is the solute. In the art, it is accepted practice to call a mixture of polymer and solvent a solution if it is optically clear without obvious inclusions of either phase in the other. Phase separation is usually then taken to be that point where there is an optically detectable separation.
There is yet another case where the heated mixture of polymer, solvent, and other components if present, is neither a solution nor a mixture in the usual sense of the words. This is the case where a surface-active agent is present in sufficient concentration to form ordered structures such as micelles.
In U.S. Pat. No. 3,378,508, a polymer is heated with a solvent that is an anionic surfactant. The solution is then cooled and membrane formation results which is in accordance with the teachings of the spinodal decomposition technique. As the anionic surfactant solvent is a solid at room temperature, solvent removal is by removal of the solid surfactant from the pores.
U.S. Pat. No. 4,247,498 describes the use of the spinodal decomposition technique with slow cooling of the solution in relation to a wide range of polymers and solvents. According to U.S. Pat. No. 4,247,498, the slow cooling of the solution-allows the solvent droplets to coalesce somewhat before the polymer freezes around them. As the solution is cooled, more and more solvent droplets are formed. Their rate of coalescence is governed by the rate of diffusion of solvent through the liquid polymer matrix. A longer time at a higher temperature allows a greater diffusion of the solvent. The slow cooling also affects the rate at which crystal nuclei form in the polymer. The result is a membrane containing large cells interconnected by fine channels. The porous material of U.S. Pat. No. 4,247,498 is extruded into air to form a block having a skin over its surface.
U.S. Pat. No. 4,564,448 discloses a porous surface achieved by the technique of extruding the polymer solution into a bath of the solvent used forming the solution with the temperature of the bath being above the temperature where phase separation occurs.
U.K. Specification 2,115,425 discloses a spinodal decomposition technique with the modification that the solvent for the polymer is a mix of solvents, one of which is a very good solvent for the polymer and the other is a poor solvent. The ratio of the two solvents is adjusted to obtain a composite solvent that has the desired temperature related solvency effect for the polymer. Variation in the ratio of the two solvents affects the structure of the resultant membrane.
The process disclosed in abovementioned U.S. Pat. No. 3,378,508 consists of heating a mixture of a solid thermoplastic polymer of mono-ethylenically unsaturated hydrocarbons, and, a water-soluble anionic surfactant to a temperature where the polymer and surfactant are mutually soluble, cooling the mixture to a temperature where the polymer and surfactant form two intermingled separate phases, and, dissolving the surfactant phase from the polymer.
According to U.S. Pat. No. 3,378,508, the hydrocarbon and surfactant are mixed at elevated temperatures at which the two materials are mutually soluble to obtain a completely uniform distribution of the surfactant in the polymer. The specification discloses that any water soluble anionic surfactants and solid, thermoplastic polymers of mono-ethylenically unsaturated hydrocarbons form a homogeneous mixture at a temperature determined by the higher of either the melting point of the surfactant or the softening temperature of the polymer.
U.S. Pat. No. 3,378,507 details attempts made to incorporate into polyethylene non-ionic and cationic surfactants such as nonylphenoxypoly(oxyethylene) ethanol, polyoxyethylated fatty alcohols, polyoxyethylated fatty acids, polyoxyethylated vegetable oil, copolymers of polyoxyethylene and polyoxypropylene, polyethylene glycol ethers and mixed alkyl amine salts containing an average of 18 carbon atoms in the aklyl group.
The non-ionic surfactants were found to be incompatible with polyethylene and would not mix with polyethylene. Although the Cationic surfactants could be milled into the polyethylene, the cationic surfactant could not then be washed out by water, ethanol or other solvents.
Contrary to the teachings of U.S. Pat. No. 3,378,507, we have found that certain cationic and non-ionic surfactants can be used to form porous materials. For example, whereas the U.S. specification states that nonylphenoxypoly(oxyethylene) ethanol cannot be used, we have found that at least some of that class of surfactant can be used.
An influencing factor is the polarity of the cationic or non-ionic surfactant which can be described in relation to the hydrophilic-lipophilic balance of the surfactant. Within a certain hydrophilic-lipophilic balance range we have found that there is no restriction on the type of surfactant than can be used to form porous membranes from a polymeric solution.